This invention relates generally to autopilots and more particularly to a marine autopilot having an adaptive deadband feature.
As is known in the art, marine autopilots are used to maintain a ship, or vessel on a fixed course while the vessel encounters environmental variations such as changes in wind speed and direction and changes in sea conditions. Preferably, the vessel course is maintained with minimum intervention by the operator of the vessel. In particular, the autopilot adjusts the position of the vessel's rudder in order to compensate for course deviations caused by changes in, inter alia, waves, wind, currents, and vessel speed.
Some marine autopilots use a proportional plus integral plus derivative (PID) control law to maintain the vessel on a desired course (i.e. during course keeping operation) and a proportional plus derivative (PD) control law to change the course of the vessel (i.e. during course change operation). Such an autopilot provides an output signal, hereinafter referred to as a rudder control signal, which corresponds to a desired change in the position of the rudder. During course keeping operation, the rudder control signal is proportional to the summation of the following terms: an error signal (i.e. the difference between a desired course and the actual instantaneous vessel heading), the time integral of the error signal, and the time rate of change of the error signal. Whereas, during course change operation, the rudder control signal is proportional to the summation of the error signal and the time rate of change of the error signal.
More particularly, each term of the conventional PID and PD control laws has a gain value associated therewith. The gain value associated with the error signal may be referred to generally as a proportional gain value, the gain value associated with the time integral of the error signal may be referred to generally as a trim value, and that associated with the time rate of change, or derivative of the error signal may be referred to generally as a counter rudder value. Thus, during course keeping operation for example, the rudder control signal (i.e. the course keeping signal) is equivalent to K.sub.p e(t)+K.sub.d e(t)+K.sub.i .intg.e(t)dt, where e(t) is the error signal K.sub.p is the proportional gain value K.sub.i is the trim value, and K.sub.d is the counter rudder value.
In such a control system, the proportional term (i.e. K.sub.p e(t)) provides rudder movement proportional to the error signal. The derivative term (i.e. K.sub.d e(t)) provides damping in the sense that once the vessel yaws, the derivative term provides resistance to such motion, or angular velocity. In this way, the derivative term reduces overshoot of the vessel past a desired course. The integral term (i.e. K.sub.i .intg.e(t)dt) provides compensation for low frequency disturbances, such as wind, by providing a bias on the rudder position to offset the effect of such disturbances. Generally, the rudder control signal provided during course change operation (i.e. course change signal) is as described above with the exception that the integral term (i.e. K.sub.i .intg.e(t)dt) is nulled, or excluded, thus resulting in proportional plus derivative control.
As is also known in the art, autopilots often include manual adjustment capabilities for modifying various control system parameters. For example, autopilots often include a manually adjustable deadband feature whereby the rudder motor is actuated only in response to rudder control signals having a value greater than an operator adjustable threshold, or deadband value. Generally, the operator or helmsman is instructed to decrease the deadband value when the vessel is heading in the direction of the waves and to increase such value in following seas (i.e. when heading away from the waves). In this way, high frequency perturbations in the vessel's heading, generally occurring when the vessel is heading in the direction of the waves, will not be compensated. Increasing the deadband value when heading into the waves is desirable since, in such conditions, movement of the rudder to compensate for high frequency heading perturbations will have little effect on maintaining the desired vessel course. In other words, a certain amount of yawing of the vessel, as it heads into the waves, is unavoidable. By allowing the high frequency perturbations in the vessel's heading to go uncompensated, ineffectual rudder movement is avoided. In this way, unnecessary wear on the rudder and associated drive apparatus is minimized.
However, manual adjustments somewhat defeat a primary purpose of an autopilot; namely, to maintain a fixed vessel course with minimal operator intervention. Further, such adjustments are not trivial and may be difficult for one unskilled or inexperienced in boating and/or autopilot operation. Thus, although the basis for providing a manual deadband adjustment is to improve the efficiency of rudder operation by eliminating ineffective rudder motion, rudder efficiency of an autopilot having a manually adjustable deadband feature may actually be degraded by incorrect or excessive adjustments of a manually adjustable deadband feature.
One technique known in the art for minimizing operator intervention with regard to manually adjustable features is to provide a fixed number of settings for the manual adjustments. In other words, a fixed number of possible deadband settings may be provided for selection by the operator of the vessel. However, the performance of such autopilots may be degraded by the lack of fine tuning capability for such adjustments.
Another technique known in the art for minimizing operator intervention with a deadband feature is to adjust the deadband value as a function of the number of reversals in the rudder's direction occurring during a fixed period of time. In this way, such an arrangement raises or lowers the deadband value as a function of rudder activity regardless of course keeping performance.